<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var obj = {
            a: 1,
            b: 2,
            c: 3
        };
        Object.defineProperty(obj, 'b', {
            configurable: false,
            enumerable: true,
            value: 22,
            writable: true,
        });

        // delete obj.b;

        obj.b = 99;


        for (let key in obj) {
            console.log(key);
        }
        console.log(obj);
        //  注意 ： 默认值
    </script>
</body>

</html>